Adds a factory function to ViewsDelegate for DesktopWindowTreeHost

ui/views/widget/desktop_aura/desktop_window_tree_host_chromeos.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/views/widget/desktop_aura/desktop_window_tree_host.h"
 
 namespace views {
 
 // static
 DesktopWindowTreeHost* DesktopWindowTreeHost::Create(
     internal::NativeWidgetDelegate* native_widget_delegate,
     DesktopNativeWidgetAura* desktop_native_widget_aura) {
-  // DesktopNativeWidgetAura is only used with mus, and MusClient injects the
-  // DesktopWindowTreeHost so that this is never called.
+  // DesktopNativeWidgetAura is only used with mus, and MusClient sets itself
+  // as the factory for both NativeWidgets and DesktopWindowTreeHosts, so this
+  // should never be called.
   NOTREACHED();
   return nullptr;
 }
 
 }  // namespace views